Most antennas for satellite communication systems require circular polarization with a good axial ratio, large frequency bandwidth and low cost. Quadrifilar Helical Antennas (QHA) have been extensively used since they are well suited for such applications as they have good axial ratio good over a broadband. In particular, they are preferred over patch antennas for handset applications where helices show better multipath rejection in the absence of large ground planes.
One of the primary design goals for handset applications is reducing the size of the antenna. For helix antennas, the reduction in size can be reducing the height or radius of the helix.
Various techniques are proposed in the literature to reduce the height of helix antennas, ranging from meanders to sinusoidal profiles. These techniques are mostly applied to antennas with radial or iso-flux patterns, whose height may be between one and two wavelengths thus demanding for some reduction. Size reductions up to 50% are reported with acceptable gain degradation. The main problem is that the length of each arm should be an odd quarter wave length multiple (i.e. nλ/4, n=1, 3, 5 . . . ) for open ended arms or even quarter wave length multiple for short ended helices. (FIG. 1). The shortest quadrifilar helix has arms with arm length equal to quarter wavelength (λg/4). However, the impedance of the antenna at this resonance is small and makes it difficult to match. The conventional length for quadrifilar helix arms is (λg/2). Therefore, a conventional way to miniaturize a helix antenna is to load it with materials with high dielectric constants which results in gain reduction and narrowband performance. Furthermore, reducing the radius of the helix also raises several problems. The most important one is that, the mutual coupling between adjacent ports increases rapidly as the helix radius becomes smaller. This eventually results in energy coupling between ports and not radiating from the antenna.